• d-bugmail puremagic.com (76/76) Aug 23 2012 http://d.puremagic.com/issues/show_bug.cgi?id=8580

d-bugmail puremagic.com writes:

http://d.puremagic.com/issues/show_bug.cgi?id=8580

           Summary: VariantN.peek works wrongly for types with size bigger
                    than maxDataSize template argument
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody puremagic.com
        ReportedBy: cybevnm gmail.com


--- Comment #0 from cybevnm <cybevnm gmail.com> 2012-08-23 16:31:48 PDT ---
Consider the code:

import std.stdio;
import std.variant;

struct S(size_t BuffLen)
{
  ubyte[BuffLen] arr;
}

void fill(T)(ref T val)
{
  foreach(ref item; val.arr)
  {
    item = 0xff;
  }
}

void printFewBytes(T)(T val)
{
  foreach(i; 0..10)
  {
    writef("%02X ", val.arr[ i ]);
  }
  writeln();
}

// for 64 bit linux maxDataSize is 32 bytes
void main()
{
  // case when user type fits directly in Variant's storage
  {
    S!(32) s; // structure with specified size
    fill(s); // fill structure with some recognizable pattern (0xFF in our
case)
    auto v = Variant(s);
    printFewBytes(*v.peek!(S!(32))());
  }

  // case when user type bigger than Variant's storage
  {
    S!(33) s;
    fill(s);
    auto v = Variant(s);
    printFewBytes(*v.peek!(S!(33))());
  }  
}

Output:
FF FF FF FF FF FF FF FF FF FF 
80 FF 02 33 4E 7F 00 00 00 00 

For structure with size smaller than Variant's storage we have expected output,
in second case we seeing some random bytes, which actually are address of
dynamically allocated by Variant's implementation object.

Some fragments of variant implementation:
// part of Variant.opAssign, where pointer saved to storage
...
auto p = new T;
*p = rhs;
memcpy(&store, &p, p.sizeof);
...

// part of Variant.peek where storage erroneously interpreted as type's
instance bytes
...
return type == typeid(T) ? cast(T*) &store : null;
...

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